Process and apparatus for the manufacture of pulp



Feb. 20, 1934. Q B, THORNE 1,947,888

PROCESS AND lAPPARATUS FOR THE MANUFACTURE OF PULP Filed Apr'il 23, 1932 a a mh' Patented Feb. 20, 1934 PATENT OFFICE PROCESS AND APPARATUS FOR THE MANUFACTUBE F PULP Carl Busch Thorne, Hawkesbury, Ontario. Canada Application April 2s, 1932. Ysei-nu No. 607,152

6Claims.

This invention relates to the prevention of loss in the manufacture of pulp. The invention is especially applicable to the manufacture of wood pulp in paper mills but may be utilized for 6 analogous purposes.

One object of the invention is to prevent loss of fibre from the blowpit during the initial part of the operation of discharging the digester into the blowpit.

Another object of the invention is to recover heat from the discharged contents of the digester during and/or after the cooking operations.

A further object is to cool the pulp and liquor as they are blown from the digester in order to obtain important advantages which will be hereinafter described.

The invention consists Fin the steps and combinations and arrangement of parts hereinafter fully described and more particularly pointed out l0 in the appended claims.

Referring now to the accompanying drawing which illustrates, by way of example, one convenient form of apparatus for carrying the invention into effect:-

Figure 1 is a diagrammatic sectional elevation of the apparatus,

Figure 2 is an enlarged section on line 2-2 of Figure 1, and i Figure 3 is a fragmentary sectional elevation 80 illustrating a modification of the invention.

1 represents any suitable digester as commonly used for cooking wood chips in the manufacture of sulphite pulp. 2 indicates any convenient recipient or separator tank into which the relief 95 acid from the digester 1 is discharged through a pipe 3. In this tank the free gases are separated from the liquor and the gases are led oi! along a pipe 4 while the liquor flows down through' a pipe 5. 6, '1 and 8 are heat exchangers consisting of a number of pipes made of acid resisting material, through which hot liquid or gas flows, these pipes being surrounded by a cooler liquid flowingin the opposite direction,

, thus absorbing the heat. from the pipes. The

heat exchangers 'are of ordinary construction and the pipes are indicated only on the exchanger 8.

A blowpit 9 is provided and may conveniently be of the type commonly used in pulp mills;

that is, of the typecomprising a tank having a perforated false bottom 10. The pulp at a high temperature is discharged from the digester 1 into'the blowpit 9 through theblowpipe l1 controlled by a valve 12. The pulp is retained in the o5 blowpit while the liquor drains down through the perforated false bottom 10 and in most mills it ows directly to a sewer through a valve controlled pipe 13. It will be readily understood that until a substantial mat of pulp has formed on the false bottom 10 the liquor which is draining away will carry with it a considerable quantity of pulp bre which will be lost. In orderto prevent this loss of fibre, the present invention provides a pump 14 and piping 15, 15 so that the first liquor which drains through the perforated iloor l0 may be pumped back into the blowpit 9. In this piping two valves 16 and 16' are provided, the valve 16 being in the part 15 of the piping, which leads back to the blowpit 9, and the valve 16' being in the part 15 of the piping leading to the heat exchanger 8. When the digester 1 is dumped the valve 16 is opened and the valve 16' closed and the pump 14 is started. The rst liquor to drain through the false bottom` I10 which carries pulp bre with it, is circulated through the ,pipe 15 back into the blowpit 9. After a few minutes a mat of fibre is formed over the false bottom 10 and this mat forms anexcellent ltering medium so that only 'comparatively clear liquor can drain through the false bottom 10. The valve 16 is now closed and the valve 16' opened. The liquor now flows up through the heat exchanger 8 and then down the pipe 1'7 to the sewer.

A further feature of the invention relates to the recovery of heat, which will now be described.

Cold water, or other liquid to be heated, for example cooking liquor, is pumped under pressure into the top of the heat exchanger 8 through a pipe 18 while the hot liquor from the blowpit 9 is pumped through the heat exchanger 8 in the opposite direction. 'I'he cold liquid to be heated ows over the outside of the exchanger pipes while the hot liquor is on the inside in the known manner. The heat from the liquor is thus transferred to the cold liquid. The liquid from the heat exchanger 8 passes through a pipe 19 to the heat exchanger 7 where it absorbs more heat from the hot liquor which is flowing through the heat exchanger from the recipient 2. The liq- 'uor from the heat exchanger 'I is allowed to go to the sewer. The water or other liquid from the exchanger 'I is carried through the pipe 20 to the exchanger 6 where it absorbs more heat from the gases from the recipient 2 as well as from the top relief gases'from the digester owing through a pipe 21. The liquid, which by this time will have reached a temperature of, say, 90 to 95 C. or more, as it leaves the heat exchanger 6, is carried through a pipe 22 to a tank, not

shown, for use as required. The gas from the heat exchangerl 6 flows along ay pipe 23 to any suitable recovery tower or other apparatus as desired.

It may be very important when water is being heated in the heat exchangers, that the water should be free from any trace of acid, and in such instances there must be no leakage of acid or gas out from the pipes into the water. When it is necessary to accomplish this, the pressure outside oi the exchanger pipes is always kept at a pressure which is greater than the pressure inside the pipes. In this way if there should be any break or leak in the pipes carrying the acid, then the liquid will lead into the acid and not the acid into the water.

To accomplish this, a pressure relief valve 24 is placed in the water line 22 Ias it leavesl the last heat exchanger 6 and this valve may be regulated so that the pressure on the water in the system is always kept at a pressure which is higher than that of the hot uids in the system. A check valve 18 may be provided in the supply pipe 18 to prevent reduction of pressure in the event of failure of the pump supplying the liquid to be heated.

In addition to this, a thermostatically con- I trolled valve 25 may also be provided in the-water line 22, a controlling thermometer 26 being placed in the heat exchanger 6. This valve 425 is of known construction and may be set to open at any desired temperature, for instance to 95 C., and will close as soon as the temperature drops below this point.

In this way both the pressure of the water, as well as the temperature of the water, are controlled as desired.

A further important feature of the invention relates to the recovery of SO2 gas from the blowpit 9. With customary blowpit construction the pulp is blown from the digester 1 through the blowpipe 11 into the blowpit 9 and all of the steam and gases escape into the atmosphere through a vomit stack leading from the top of the blowpit. With this present system most of the gases are recovered, as well as most of the heat in the steam. To accomplish this means are provided for cooling the pulp at the point' around the blowpipe where it enters the blowpit. In the illustrated form of the invention, these means comprise a cylindrical jacket 27, the inner wall of which is perforated. Cold water enters the jacket 27 through a pipe 28 and passes through the perforated inner wall 27' into the pulp as it enters the blowpit.

The temperature of the pulp and 4liquor in the digester just before it is dumped may, for example, be approximately 145 C. at 70 lbs. pressure. Thus, if this pulp and liquor are cooled as they leave the digester the pressure is reduced, together with the temperature, resulting in much less expansion of the gases and steam, so that the liquids from the digester, instead of ashing into vapour as soon as they leave the blowpipe, remain to a much greater extent in a liquid form, and instead of creating a pressure in the blowpit, they flow down through the perforated is added through pipe 28 the SO2 gas will be absorbed and will'low down through the perforated false bottom and be lost. Also, if insuflicient cold water is admitted throughthe pipe 28, then the steam is not condensed and so the heat would not be recovered in the heat exchanger 8. Therefore, the size of the water jacket, as well as the quantity of water supplied, must be very accurately determined.

The SO2 gas which is liberated in the blowpit is carried out through a pipe 29 to any suitable gas absorption and recovery system, nnt shown.

It will, of course, be understood that the arrangement shown on the drawing is only diagrammatic and may be changed considerably without departing from the invention. -For instance, the heat exchangers may be arranged differentIy, or heat exchangers 6 and 7 may be combined into one unit, or a larger number of heat exchangers may be used to obtain the same final result.

The recipient 2, as shown, is not essential, but is illustrated as a preferred construction. If no recipient is used, as is the case in some mills. then the gas,- together with the liquor, may pass through the whole system of heat exchangers.

Among the many advantages of this feature of the invention, the following maybe mentioned:-

1. Heat which, up to now, has been wasted in the blowpits is recovered.

2. The gases which are liberated in the blowpit are recovered. 4

3. The necessity of a vomit stack is eliminated, thus saving considerable expense.

4. The disagreeable and sometimes destructive odors and gases are not emitted into the atmosphere.

The foregoing description and accompanying drawing are given by way of illustration only, and any modications may be resorted to within the scope of the appended claims without departing from the invention.

For example, instead of using the water jacket 27 and the SO2 exhaust pipe 29, the gases may 120 be cooled and condensed in the manner indicatedl in Figure 3. A cooling tower 30 is placed over the blowpit, which tower is of rectangular, round, or other cross section and has inside of it a number of nozzles 3l, which spray cooling liquid 125 onto the gases and vapours as they rise. At the top.- of the tower is an opening 32 to which is connected the suction of'a fan 33.

In operation, the digester is blown in the usual manner and the steam and gases which usually go out through the vomit stack are drawn up through the cooling tower 30 by the fan 33. The gases and vapours come in contact with the cold water sprays from the spray nozzles 31. The steam is condensed and falls back into the blowpit 9 while the gases are sucked up through the opening 32, through the fan 33, and out through a pipe 34, to any suitable gas absorption and recovery system.

In mills wherein the hot liquor is not drained into a blowpit, the heat may be recovered by means of an arrangement such as that indicated in dotted lines to the right of Figure 1. In this case, the gas which is given oi during cooking is relieved through the pipe 21, and the excess cooking liquor is relieved through the pipe 3, as already described. Now, instead of opening the valve 12 in the blowpipe 11 and dumpingV the whole charge of the digester into the blowpit, a valve in the pipe 21 is fully opened and the gas 150 is allowed to escape through the heat exchanger 6 until the pressure is reduced practically to at-` ytop of the digester to wash the pulp in the digester, the water descends and leaves the digester through the pipe 15a. In this way the heat from the liquor is recovered in the heat exchanger 8B. After the pulp is washed inside the digester, the valve 15b is closed, and the valve 12 in the pipe l1 is .opened vand the pulp is allowed to discharge into the blowpit 9 or into any other tank for further processing in the mill. 'Ihe liquid to be-heated enters exchanger 8* through a pipe 18a which may be fitted with a check valve 18", and, after being heated in the exchanger, passes on through pipe 19n to exchangers '7 and 6 to be further heated as hereinbefore described. When this arrangement is employed the exchanger 8, pipes 18 and 19 and cooling device 27 are omitted.

I claim:

1. In the manufacture of pulp, discharging liquids and solids from a digester into a blowpit, allowing the liquids to drain from the blowpit, and pumping said liquids back into said blowpit until a sufficient quantity of the solids has accumulated in the blowpit to form a filter for the liquids.

2. In the manufacture of pulp, discharging liquid and nbre-containing solids from a digester onto a straining device in a blowpit, allowing the liquid `to drain through said straining device and out of the blowpit, and pumping said liquid back into said plowpit above the straining device until a mat of the solids has accumulated on said straining device.

3. In the manufacture of pulp, discharging liquids and solids from a digester into a blowpit, allowing the liquids to drain from the blowpit, pumping said liquids back into said blowput until a sufiicient quantity of the solids has accumulated in the blowpit to form a filler for the liquids, and thereafter pumping the liquids through a heat exchanger.

4. Apparatus for manufacturing pulp, comprising a digester, a blowpit having an outlet and a connection to said digester, and means connected to said outlet for returning materials passing through said outlet to said blowpit when desired.

5. Apparatus for manufacturing pulp, comprising a digester, a blowpit having an outlet and a connection to said digester, a straining device in said blowpit to receive materials discharged from said digester, a heat exchanger connected to said outlet, and a pump for forcing materials from said outlet alternatively through said heat exchanger or back to said blowpit.

6. Apparatus as claimed in claim 5, wherein an automatic thermostatically controlled valve is provided to stop the circulation of the liquid to be heated when the temperature of said liquid falls below a predetermined limit.

CARI.. BUSH THORNE.

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